1. Field of the Invention
The present invention relates to the field of communication via electrical power supply networks. It is based on an apparatus for injecting signals into electrical power lines.
2. Discussion of Background
The use of existing power distribution networks for data communication is of major economic importance. Electricity supply utilities would like, for example, to interrogate devices in the electrical power distribution network without reinstallation of lines and control them remotely, or optimize the utilization of the network by remote interrogation and control of end users.
German Patent Specification DE 196 48 382 C1 discloses a method and an apparatus for transmitting signals via electrical power supply lines, in which communication signals are transmitted via the phase conductor shields of electrical power cables. The signals are modulated onto the shields via inductive couplers. This method is obviously applicable only to electrical power cables having shields, and is unsuitable for unshielded cross country lines. Furthermore, the circuit for a signal transmission device is described, in which the communication signals are infected and/or output via a coupling capacitor to and/or from at least one phase conductor of the electrical power line.
The coupling capacitor (KK) is connected to a phase conductor at the medium-voltage or high-voltage level by means of a connecting cable (AK) through which no current passes. The AK and KK are normally connected outside a housing which surrounds the KK and is grounded. The bushing (which is compatible with medium voltage or high voltage) through this housing comprises means for field reduction, in order to avoid field peaks. A suitable way to achieve this is, for example, by means of a field-controlling layer composed of a material having a high dielectric constant, which layer is arranged as a flexible tube or strip around the electrical power conductor (cable termination). A capacitive field controller disclosed in WO 80/00762 is suitable for bushings through grounded metal walls. This is based on a number of conductive metal foils arranged concentrically around the central conductor.
The connection of the connecting cable to the bushing requires a high level of insulation complexity and occupies a large amount of space. Furthermore, it is not feasible in modern compact switchgear assemblies owing to the excessively short isolation spacings with conventional cable terminations. As a substitute, connections by means of insulated high-voltage coaxial connectors are feasible, but these are expensive and complex to install.